In certain ultrasound imaging systems, pulses of several sinusoidal pressure waves are successively transmitted by each transducer of an array into a body under examination so as to form a beam along a path in a direction determined by the relative times at which the transducers of the array are excited. Reflection of these pressure waves from targets located in the beam impinge on the individual transducers and cause them to produce corresponding electrical waves. Because of the different distances between a target and each of the transducers, the electrical waves they produce in response to pressure waves reflected from the target are not in phase, so that summing the electrical waves would produce a signal of very low amplitude. If, however, a delay is connected to each transducer that exactly compensates for the differences in time it takes for the reflections to go from the target to each transducer, corresponding sinusoids of the pulse will be cophasal, and a summation of these cophasal waves will produce a strong signal. Because a different delay must be provided for each target range along each beam path, the number of delays required is great and their cost is excessive.
In U.S. Pat. No. 4,140,022 issued on Feb. 20, 1979, to Samuel B. Maslak and assigned to Hewlett-Packard Company, an ultrasound system is described in which the cost of providing the delays is considerably reduced. Instead of providing the many delays that are required to cause the pulses of electrical waves produced by the transducers to occur at the same time so that corresponding sinusoids within the pulses are all in phase, fewer delays are provided so that, although the pulses do not arrive at the same time, they do have some overlap. Each transducer is connected to one input of its own mixer and the output of the mixer is connected by a tap selector to a tap on a master delay line. The other input of each mixer is connected to a phase selector that provides the phase of a common local oscillator signal that is required to bring the sinusoids of the intermediate frequency at the outputs of the mixers which occur during the overlapped portions of the pulses into approximate phase coherence at the output of the master delay line. A strong signal is attained when these signals are summed. In order to secure the required degree of phase coherence, however, it is essential that the tolerance of the locations of the taps on the master delay line be such that errors in the delays provided by the taps are within a small fraction of a period of the intermediate frequency wave of the delay they are supposed to provide. This is so because the phases of the intermediate frequency provided by the mixers is based on the assumption that the locations of the taps are ideal.